In a typical radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for e.g. third generation networks and further generations, and investigate enhanced data rate and radio capacity.
Specifications for the Evolved Packet System (EPS) have been completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio base stations without reporting to RNCs.
Currently a 4Tx transmissions scheme for High Speed Downlink Packet Access (HSDPA) is discussed within 3GPP standardization [1], [2]. Radio communications networks for packet based communication often use a retransmission protocol such as hybrid automatic repeat request (HARQ) on a physical layer to achieve greater reliability and robustness against the impairments of the radio channel. HARQ combines forward error correction (FEC) with automatic repeat request (ARQ) by encoding an information containing data block, also known as transport block (TB), in an encoder and then adding cyclic redundancy check (CRC) bits or other error detection bits to the coded bits output from the encoder. The coded data block is referred to as a codeword. If the data block is received without error, an acknowledgement (ACK) is sent to the transmitter indicating successful transmission of the data block and a new data block is transmitted. On the other hand, if the data block was not decoded correctly, a negative ACK (NACK) is sent by the receiver to request a retransmission. One fundamental issue with the four branch Multiple Input Multiple Output (MIMO) system is how many codewords/HARQ process identifiers this MIMO system should support. To reduce the signaling in uplink and downlink the radio communications network uses two HARQ process identifiers. Four codeword MIMO and two codeword MIMO are herein described in details. Note that codeword means, the blocks which have the same HARQ process identifier, same modulation and same transport block size, or code rate. It may be two transport blocks having the same HARQ process identifier and which are allocated same modulation and code rate, i.e. Transport block size.
Four Codeword MIMO
FIG. 1 is a block diagram of four codeword-four branch MIMO system and shows the transmission block diagram of a four codeword four branch MIMO system. This is an extension of Release-7 MIMO for HSDPA to four layers corresponding to the four branches. From the feedback information from the UE, the adaptive controller at the Node-B chooses the transmission rank (R), i.e. number of transmission layers, transport block length, modulation order and coding rate. Based on the feedback information taken into account in an adaptive controller, it also generates the precoding weight matrix (P). Observe that for this scheme coding and modulation is done independently for each stream. Hence one transport block (TB) is mapped to one codeword.
The information bits from the transport blocks are passed to the channel encoder and the output is interleaved and modulated in an interleaver and modulator for each stream. The output of the interleaver and modulator is mapped to the layers directly, i.e. symbols from each codeword are directly mapped to the corresponding layer. Once the layer mapping is done, the resultant symbols are spread and scrambled in a spreader and scrambler. Precoding is applied on the output of the spreader and scrambler in a precoder and the output signal is passed to the corresponding antenna ports.
The receiver transmits feedback of the channel to the transmitter wherein capacity maximization is found by summing up all the layers, which is optimal for four codeword MIMO system, however, summing of capacities may not be optimal in a MIMO system using a different number of codewords or a different relation between codewords and number of transmission layers.